1. Field of the Invention
This invention relates to the reformatting of composite video signals into a symmetrical form which may be easily sampled and manipulated in a spatial or geometric sense, for such uses as graphic overlay, zoom, pan, windowing, etc., while still in a composite form.
Of the popular worldwide color composite video standards, National Television Standards Committee (NTSC), Phase Alternating Line (PAL), and Sequential with Memory (SECAM), each have tradeoffs and features which make them appropriate for broadcast and reception by either black & white or color television receivers. The NTSC composite color video signal, for example, is formed by taking the overall intensity (luma) portion of the subject video and adding to it a color (chroma) subcarrier signal of 3,579,545 Hertz (Fsc) which has been modulated in phase in response to color hue and in amplitude in response to color saturation. The PAL composite video standard uses a chroma subcarrier frequency of 4,433,618.75 Hertz. A portion of the unmodulated chroma subcarrier signal called "color burst" is placed on each video scan line as a chroma phase reference. The PAL composite video standard reverses the phase of this reference signal with each successive scan line. The resulting composite signal may be broadcast over a single channel and subsequently displayed on either a color television receiver by demodulating the color information, or on a black and white television receiver by displaying the entire composite video signal as if it were a black and white only video signal. It is this latter black and white compatibility requirement which gives standard composite video a characteristic non-symmetry with respect to line-to-line and field-to-field chroma reference phase, although non-linear chroma demodulators also benefit from the averaging effect of this phase irregularity.
Orthodox NTSC composite color video signals have a horizontal scan line frequency equivalent to 227.5 cycles of chroma reference (Fsc), resulting in a 180 degree phase alignment shift of the chroma reference from line to line. If an integral number of chroma reference cycles were to occur for each scan line, the chroma subcarrier peaks and valleys for a given color would align vertically and highly saturated colors would show as vertical bands on the screen of a black and white television. As it is, the 180 degree phase shift causes the alternating peaks and valleys to appear as a fine checker-board pattern which is visually more pleasing. The checker-board pattern is further blurred by forcing an additional 90 degree phase shift on each subsequent video field. Therefore, only every fourth field has the same line to reference phase relationship.
While composite video signals are a very compact and efficient format for storing, transmitting, and recording color raster video signals, the irregular relationship between scan lines and chroma subcarrier phase makes spatial manipulation of such signals very difficult and inefficient. By way of example, consider known methods of sampling a composite video signal into a field memory and subsequently playing it back to a video screen, providing a so-called "freeze-frame" capability. FIG. 1 shows such sampling within a typical such NTSC system, wherein the original chroma reference is reproduced phase-locked to the color burst signal within the composite video signal and sampling of the composite video signal occurs at three specific phase points with respect to the reproduced chroma reference, for a sampling frequency of 3 Fsc. While any sampling rate above about 2.5 Fsc may be used, an integral sampling rate allows samples to be easily repositioned spatially within the video field without loss of hue information. This is because the hue is represented as the phase difference between the chroma reference and the modulated chroma subcarrier within the composite video signal. That phase relationship extant in any given sample set ABC will be valid anywhere in any field where the same sample-to-reference phase relationship exists. The three sample values representing a particular shade of yellow, for example, will be the same values when that shade of yellow is sampled anywhere else in any field, as long as the samples are taken at the same phase points, for example the zero degree point, the 120 degree point, and the 240 degree point, with respect to the chroma reference.
FIG. 2 shows a typical layout of a field memory as it relates to the physical aspects of an NTSC video field. Individual video sample sets are stored in individual memory locations or addresses. Where spatial manipulation of the video is contemplated, the video is stored for geometric convenience as a series of independent scan lines with each line beginning at a horizontal reference point. This scheme allows for efficient location a given geometric coordinate as a particular row (scan line) and column (sample set) within the field memory. The horizontal reference point is set at the first sample set after horizontal sync is detected in the video. FIG. 2 also shows an expanded view of a small portion of the video field with associated memory addresses aligned as necessary to sample sets. The expanded view shows a segment of three scan lines. As graphically depicted, address 206 contains the sample set for scan line 2 (the vertical address component 2xx), sample set 6 (the horizontal address component x06). Similarly, addresses 305 and 306 contain samples sets 5 and 6 respectively for scan line 3, and address 406 contains sample set 6, line 4. The staggered alignment of the addresses between scan lines illustrates the actual location of the samples sets with respect to the rectangular raster of the displayed video field. As discussed above, this stagger is caused by phase-synchronous sampling relative to the chroma reference which reverses phase on each successive scan line.
FIG. 3 shows a larger portion of the subject field memory in the vicinity of the horizontal reference, with the impact of the horizontal reference on the memory addressing. Horizontal address components are shown in hexidecimal numbering. Also shown is the position of an example vertical video detail (such as a telephone pole) in the video field. Video values representing the vertical detail would be sampled in the sample set and individual sample which intersect the vertical detail. As shown in FIG. 3, the vertical detail would be reflected in: line 2, sample set 3, (or address 203) sample C; line 3, sample set 3, (or address 303) sample A; line 4, sample set 3, (or address 403) sample C; line 5, sample set 3, (or address 503) sample A; line 6, sample set 3, (or address 603) sample C; and line 7, sample set 3, (or address 703) sample A. A "window" surrounding the vertical detail, including addresses 202, 203, 204, 302, 303, 304, 402, 403, 404, 502, 503, 504, 602, 603, 604, 702, 703, 704, etc. is defined for an example of spatial manipulation.
The above discussions have related to orthodox NTSC composite video sometimes referred to as "studio quality" NTSC composite video because the horizontal line frequency is scrupulously maintained at 2 Fsc divided by 455. However, with video sources such as consumer video cassette recorders (VCRs) and video disk players, where horizontal frequency is affected by small irregularities in drive motor speed, the phase relationship between the chroma reference and the scan lines will also reflect these irregularities. The result is unorthodox NTSC composite video, sometimes referred to as having time-base error. FIG. 4 shows the example of FIG. 3, but with time-base error illustrated by the irregular line-to-line phase pattern. The phase irregularities cause the vertical detail to occur in different samples than with a regular phase relationship. In the example shown in FIG. 4, the vertical detail would be stored in: address 203, sample C; address 204, sample A; address 303, sample A; address 403, sample C; address 503, samples B and C; address 603, sample A; and address 703, sample B.
Turning now to spatial manipulation of the video field as reason for storing the video samples in a field memory, suppose that it is desirable to move the previously defined video window surrounding the vertical detail up one scan line as a video effect. The most efficient method by which to accomplish this movement is to effectively move the samples within the field memory up one logical scan line. Physically this can be accomplished by either actually moving the sample sets within the memory storage area or by manipulating the memory addressing within the windowed address space. FIG. 5 and FIG. 6 show the result of moving the vertical detail in FIG. 3 and FIG. 4 respectively, up one scan line. Since the like numbered sample sets of the next line up occupy different absolute horizontal positions within the video field, the vertical detail, while up one scan line, is no longer a purely vertical detail but rather jagged in appearance. The color of the vertical detail will be correct because the samples are in proper phase with respect to the chroma reference. With the "studio quality" composite video of FIG. 3, moving the window up one more scan line would return the sampling alignment to that of its original alignment two lines below because phase alignment is the same for every other scan line within a video field. However, with the time-base irregularities of FIG. 4, there is no guaranteed other scan line set which repeats the phase alignment of any other given scan line set, resulting in a potentially jagged video detail no matter to which other line the window is moved. Also, with the 90 degree phase shift of the chroma reference between adjacent video fields, any such moving of the video window to another such adjacent video field, even moving to the same scan line, will result in horizontally shifted video detail regardless of whether the composite video is of "studio quality" or not.
The prior art of video spatial manipulation has resolved the problem of jagged video detail by abandoning composite video as the medium for storage and manipulation, in favor of so-called "component" video. The process involves decoding the composite video into, for example, luma (Y) and chroma (R-Y, B-Y) components or primary color (R, G, & B) components, storing and manipulating the video in this form, and then encoding the components back into composite video for final output to a television receiver or monitor. In this component form, the video hue is no longer phase encoded and therefore, may be aligned horizontally in an absolute sense and manipulated freely without regard to phase-induced positional anomalies or loss of color integrity. However, this solution is inefficient with respect to video storage since the signals representing each of the components must be stored and manipulated as separate variables.